1. Field of the Invention
The present invention relates to a multipole magnetic flywheel for a hand-driven magneto megohmmeter with step variable voltage, comprising a rotor and a stator disposed inwardly of the rotor, said rotor including a ring of a magnetic material and an even number of permanent magnets carried by said ring and having magnetic axes which are angularly equidistant and disposed radially with respect to the axis of rotation of the rotor, each permanent magnet having a free pole face facing towards said axis of rotation, and said stator comprising an assembly of at least four angularly equidistant magnetic circuits, each magnetic circuit having a C-shape with a central portion having a coil disposed around it, and with end portions shaped so as to form a pole surface at each end, the pole surfaces of the magnetic circuits being disposed on a cylindrical surface situated close to the free pole faces of the permanent magnets.
2. Description of the Prior Art
Multipole magnetic flywheels having a structure similar to that indicated above, but for the ignition of internal combustion engines are for example described in French Pat. No. 2 386 925 and in British Pat. No. 979 956. In the British Pat. No. 979 956, all the coils do not have the same function, one of the coils being used for producing the ignition current, while the other coils are used for producing current for light or other sundry purposes. In the multipole magnetic flywheel described in French Pat. No. 2 386 925 and intended for use in an auto-cycle, the voltage supplied to a given load and depending on the speed of rotation is approximately regulated by the current-limiting effect provided by the leakage inductance of the coils depending on the frequency.
In the case where the magnetic flywheel is intended to supply a crank magneto megohmmeter, greater stability of the generated voltage is required. It is known to use for this purpose a centrifugal clutch connecting the crank to the magnetic flywheel as long as a predetermined speed of rotation has not been reached, and disconnecting the magnetic flywheel from the crank as soon as the desired speed has been reached.
Furthermore, to avoid having to turn the driving crank too fast, a step-up gear-train is provided between the crank and the clutch.
Finally, for some improved megohmmeters, it is required to be able to carry out insulation resistance measurements at different voltages, for example from 500 to 5000 Volts.
The presence of several magnetic circuits then allows the voltage to be varied by steps by means of a step-switch, and the independence of the magnetic circuits enables the air gaps of each circuit to be adjusted separately, so as to adjust each of the voltages to the desired values.
There appears however a difficulty due to the starting torque. It is known that a slot effect exists between the projecting poles of the inducting magnets and the edges of the pole surfaces of the magnetic circuits. The result is that the crank must supply a starting torque proportional:
-on the one hand, to the product of the total magnetic flux generated by the magnets multiplied by its angular derivative,
-and on the other hand, to the step-up ratio of the gear-train.
This torque must be limited for easy use of the megohmmeter and also to avoid damage to the gears.
There may also occur slipping of the clutch before succeeding in driving the rotor.
In order to reduce the slot effect, it is known to have an armature with oblique slots with respect to the inductor, but this arrangement is costly and causes a loss of power.
French Pat. No. 2 392 525 indicates a means for reducing the starting torque, but this solution is not applicable in the present case.